If I understand the problem correctly then there is no way for IndirectSound to work around Microsoft's bug,
because even if I manually set the mastering voice to have more than two channels
XAudio2 would still end up converting its output based on how many speakers
XAudio2 thinks that the device is outputting to.

One potential fix is this hack
discussed in the thread linked above,
but it requires you being comfortable with manually changing your local XAudio2 DLL

Missing Features

EAX ("Environmental Audio Extensions") is a collection of extensions to DirectSound
created by Creative Technology
which provides features that were used by many older games,
and EAX support is often required to experience a game
the way that its creators intended.
It is not a feature of Microsoft's DirectSound API, however
(hence the name "extensions"),
and is not required for 3D positional audio.

The future of EAX emulation in IndirectSound is uncertain.
Currently EAX support is emulated,
meaning that IndirectSound pretends that EAX is supported
and games will respond accordingly,
but there is no actual audible difference when EAX is enabled or disabled.
Doing true DSP emulation is quite a bit more difficult,
and even if the correct effects were implemented it wouldn't be true emulation
since Creative's algorithms are (understandably) proprietary.

Significant progress was made towards true EAX emulation,
but some difficult and frustrating bugs got in the way.
Very little progress has been made in the past few years,
but the hope and intent is to still be able to release something eventually.